Many methods for the modulation of light exist within the art, each method having advantages and disadvantages for each particular use. For example, a mechanical shutter may be used as a very effective means to modulate light, with near infinite contrast ratio, but the switching speed of a mechanical shutter is for the most part limited to slower than a kilohertz unless more exotic piezoelectric shutters, requiring expensive power supplies and fairly elaborate setups, are utilized. A liquid crystal modulator may be utilized for switching speeds in the low kilohertz range, and these modulators generally have good contrast ratio (typically on the order of 20 or 30:1 but some claim 500:1), however liquid crystal modulators, for the most part can only transmit polarized light and they experience photochemical breakdown when used in UV applications. An acousto-optic modulator, when used with a laser, can generally provide excellent contrast ratio (&gt;800:1) and switching speeds in the megahertz range, however its overall transmission efficiency is somewhat low at high modulation speeds, the set-up of the modulator is rather involved, and for UV operation the light must be polarized. Electro-optic modulators, when used with a laser, provide good contrast ratio with very high modulation frequencies but require polarized light, precise set-up, and their reliability is questionable.
Other modulation devices, related to the invention described herein, are; by Walles (U.S. Pat. No. 4,260,225, Apr. 7, 1981) which involves changes in solubility of a polymeric solution in a solvent, due to temperature changes, causing changes in optical density of a cell; by Waring, Jr. (U.S. Pat. No. 3,951,520, Apr. 20, 1976) which involves changes in a dispersion of two immiscible phases, due to changes in temperature, causing changes in the scattering of light through a cell; by Mattis (U.S. Pat. No. 3,664,726, May 23, 1972) which involves the transition of a metallic oxide or salt, contained in a cell, from a translucent to a reflective state, caused by heating with an electromotive force; and by Herbert (U.S. Pat. No. 4,148,563, Apr. 10, 1979) in which the total internal reflectance of a cell is changed by the refractive index change when a liquid/vapor phase change occurs.
More closely related is art by Nishimura (Japanese Application Numbers Sho 57-102305, 57-102295, 57-102296, 57-102291, and 57-102292 all filed on Jun. 16, 1982) in which a bubble is generated, by electrical resistance heating, in an opaque fluid thereby changing the cell from optically opaque to transmissive in the region of the bubble. Another application by Kawamura (Japanese Application No. Sho 60-51010, May 14, 1985) exhibits an optical shutter that works on substantially the same principle as that of the Nishimura devices.
In each of the applications described in the previous paragraph, the heat that produces the bubble is generated by an electrical element. For a transmissive cell, this electrical element must be transparent and must be accessed by a multiplexing circuit that is preferably transparent. In addition these transparent electrical elements and circuits are preferably of good optical quality in order to eliminate scattering and distortion of the transmitted light. It is also required that the electrical elements be placed in discrete locations thereby preventing the generation of bubbles in random locations within the cell. This discrete location of each electrical element requires that the elements be small and tightly packed in order to obtain high resolution of the transmitted or reflected light through the bubbles that make up an image.
Existing optical modulators generally have disagreeable characteristics in imaging applications especially where UV radiation is utilized. For example, suppose a HeCd laser is to be utilized for exposure of a photopolymer in a solid imaging or stereolithography process. The commercial HeCd lasers have a UV output of 325 nm and are fairly low in power, making the three-dimensional object formation relatively slow. To speed up the object formation process, medium power HeCd UV lasers are employed. However, to gain the higher power, manufacturers usually provide lasers with unpolarized and multimode output. In conjunction with the medium power lasers, a method of moderately fast beam modulation should be employed to ensure uniform exposure over the image plane. Unfortunately, for such a system, none of the existing modulation systems work well considering the laser output and the modulation speeds required. The mechanical shutters operate too slowly to provide the proper exposure control and even image edge control. The liquid crystal shutters require polarized light and the liquid crystal medium is not stable under UV radiation. The AO and EO modulators require polarized light for operation at this wavelength and the multimode output of the laser makes the AO modulator inefficient. If one wishes to provide a UV exposure system, a higher power UV laser, such as an Argon Ion laser may be effectively utilized with say an AO modulator, however, this entails significant added expense. On the other hand, one might attempt to utilize an incoherent UV light source masked by silver halide films to project the image. This may provide high resolution within a layer but subsequent layers may not be properly registered with other layers and the films are difficult to handle with sufficient speeds.